Field of the Invention
The present invention relates to a touch screen, a touch panel, a display device, and an electronic apparatus.
Description of the Background Art
Touch panels are devices for detecting touches by fingers and the like and for identifying positional coordinates of the touched positions. Attention has been focused on such touch panels as one type of excellent user interface means. Currently, touch panels of various types such as resistive-film types and capacitive types have been fabricated as products. In general, a touch panel includes a touch screen incorporating touch sensors, and a detection device for identifying coordinates of touched positions based on signals from the touch screen.
As one type of capacitive-type touch panels, there have been projected-capacitive type touch panels (refer to Japanese Patent Application Laid-Open No. 2012-103761, for example). Such a projected-capacitive type touch panel is capable of detecting touches, even in cases where the touch screen incorporating touch sensors is covered at its front surface side with a protective plate such as a glass plate with a thickness of about several millimeters. Touch panels of this type have excellent stiffness, because protective plates can be disposed on their front surfaces. Further, such touch panels are capable of detecting touches even by gloved hands. Further, such touch panels have longer lifetimes, because they have no movable portion which mechanically deforms.
A projected-capacitive type touch panel includes a first series of conductor elements formed on a thin dielectric film as detection wirings for detecting capacitances, and a second series of conductor elements formed on the first series of conductor elements with an insulation film interposed therebetween. Note that the respective conductor elements are not in electric contact with each other and three-dimensionally intersect with each other at a plurality of positions. Capacitances formed between an indication body such as a finger and the first series of conductor elements and the second series of conductor elements as the detection wirings are detected by a detection circuit, in order to identify positional coordinates of the position touched by the indication body. This detection method is generally referred to as a self-capacitance type detection method (refer to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 9-511086 (1997), for example).
Further, for example, there are detection methods for identifying coordinates of touched positions, by detecting changes of electric fields, namely changes of mutual capacitances, between a plurality of column-direction wirings provided to extend in a column direction and a plurality of row-direction wirings provided to extend in a row direction. This detection method is generally referred to as a mutual capacitance detection method (refer to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-526831, for example).
Any of structures of the aforementioned self-capacitance types and mutual-capacitance types generally employ methods for identifying coordinates of touched positions, based on a balance between a detected value from a sensor block and detected values from detection cells therearound, if an indication body such as a finger touches a planar area (detection cells) sectioned in a lattice shape by column-direction wirings and row-direction wirings.
Generally, sensor capacitors are constituted by column-direction wirings and row-direction wirings, and it is desirable that the plurality of sensor capacitors is fabricated to have the same capacitance, ideally, in states where no physical quantity is exerted thereon. However, in structures having a touch panel combined with a display element such as a liquid crystal display panel, an extra parasitic capacitance is formed between an outer lead wiring in the touch screen and the display element such as a liquid crystal display panel. This causes the sensor capacitor (capacitance) corresponding to the outer lead wiring to have a different offset from the offsets of the sensor capacitors (capacitances) corresponding to the other lead wirings, in states where no physical quantity is exerted on the touch screen.
It is hard to differentiate such a capacitance offset which exists independently of physical quantities exerted thereon, from a difference in capacitances induced by physical quantities exerted thereon, based on the output voltage from the projected-capacitive type touch screen. This causes errors in detecting physical quantities. For coping therewith, there have been suggested methods for reducing variations in offsets of capacitances of a plurality of sensor capacitors.
Japanese Patent No. 5106471 discloses a touch screen which includes a plurality of lead wirings, and a dummy lead wiring provided along an outer end of at least one lead wiring, out of the outermost lead wirings in the opposite sides of the wiring bundle constituted by the aforementioned plurality of lead wirings. This document discloses that it is possible to suppress the variation in the capacitance offsets, because a predetermined electric potential is applied to the dummy lead wiring.
The technique in Japanese Patent No. 5106471 can suppress the capacitance offsets, in cases where the electric potential at the dummy lead wiring is almost equal to the electric potentials at the lead wirings. However, with this technique, the electric potential at the dummy lead wiring may be made different from the electric potentials at the lead wirings, in some cases. In such cases, the capacitance offsets have been occasionally increased due to coupling between the dummy lead wiring and the lead wirings.